2.2 Measuring Earthquakes

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Vocabulary

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focus: the area under the Earth's surface where rock that is under stress breaks, causing an earthquake
epicenter: the area on the surface that is above the focus
seismic waves: vibrations that travel through Earth carrying energy released during and earthquake
P waves: earthquake waves that compress and expand the ground
S waves: earthquake waves that vibrate from side to side as well as up and down
surface waves: they move more slowly than P waves and S waves, but theground movements
seismograph: records the ground movement caused by seismic waves as they move through the Earth
magnitude: a measurement of earthquake strength based on seismic waves and movement along faults
Mercalli Scale: rates earthquakes according to their intensity
Richter Scale: a rating of the size of seismic waves as measured by a particular type of mechanical seismograph
moment magnitude scale: a rating system that estimates the total energy released by and earthquake

Outline

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Seismic Waves

• sound waves carry force as they move away from their origin
  • during earthquake, seismic waves come out from focus in every way
    • three categories for seismic waves; P waves, S waves, and Surface waves

Primary Waves

• first waves to come in from core
  • makes buildings become bigger and smaller
  • can travel through solids and liquids

Secondary Waves

• comes after P waves
• moves ground back and forth
  • shakes buildings violently
  • can't go through liqueds

Surface Waves

• when P waves and S waves reach surface, they turn into surface waves
  • moves slower than P waves and S waves
  • has the strongest ground movements

Detecting Seismic Waves

• to measure seismic waves, scientists use seismographs

Measuring Earthquakes

• the three types of scales are...
  • the Mercalli scale
  • the Richter scale
  • the Moment Magnitude scale

Mercalli Scale

• was invented on early 20th century
• it doesn't give the exact measurement
  • there are 12 steps that describe how earthquakes affect people, buildings, and land surface
  • the same type of earthquake can have different Mercalli ratings because it is in a different location

Richter Scale

• invented in 1930's
  • used for about 50 years
  • provides correct measurements for small and close earthquakes
    • scale does not work well for large or far earthquakes

Moment Magnitude Scale

• used to measure earthquakes of all sizes, both close and far away earthquakes

Locating the Epicenter

• use seismic waves to find the epicenter
  • to find epicenter, measure between arrival times of P waves S waves
    • the farther the epicenter, the farther the amount of time there is between the arrival times of the P waves and S waves

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